1. Field of the Invention
This invention relates to etching methods.
2. Discussion of Prior Art
Various techniques are currently used for etching substrates. These tend to rely either on masking areas of the substrate before exposing the substrate to an etching fluid (e.g. an acid), or etching by exposing areas of the substrate to a beam of particles or radiation.
These techniques all have drawbacks. Specific previously proposed techniques, together with their associated problems, are:
i) Mechanical sawing and milling: this is very limited in precision (blades and tools are normally greater than 100 microns) and thus are not suitable for making fine structures. They also lead to rough surface walls in the resulting structure.
ii) Particle beam and ion beam damage: in these techniques the sample is bombarded by ions resulting in the formation of a damage region in the sample. The damage region exhibits enhanced etching using a subsequent chemical etch. Such techniques are limited to make shallow structures due to limited penetration depth and uncertain straggling range. Repeated applications of the ion beams cannot be used to make deeper structures because of vignetting by the previous etching steps.
iii) Masking followed by etching: in this a mask layer (which could be photolithographically defined) results in some regions being exposed to etchant while other regions are protected. Such techniques are limited by undercutting, thus limiting the resolution and aspect ratio.
iv) Direct photon ablation (laser ablation): this can be used to make structures but leads to debris formation, imprecise features, cracking, melting and is limited to shallow structures and small aspect ratios (ratio of depth of feature to width of feature).
v) Laser imitated chemical reactions: this term includes several such techniques include sub-band-gap laser micro-machining and ultra-violet laser induced desorption, laser photo-thermal decomposition. The laser interacts directly with the material to melt, decompose or dissociate the material. It suffers from the same drawbacks as (iv).
vi) Reactive ion etching and reactive ion-beam etching: in these techniques ions are generated and impact a previously masked surface to remove material by direct impact or chemically assisted reactive species formation. These techniques are limited to shallow features, are expensive and have limited throughput. They do not exploit the underlying crystal morphology to give atomically smooth and controlled features.
vii) Ion beam milling: this technique, which is similar to (vi), uses an electromagnetically steered beam to control the pattern of etching. It has limited throughput, limited depth and can result in substrate faceting and re-deposition of sputtered material.
viii) Plasma, sputter and dry etching techniques: these are similar to (iii) but use a plasma environment instead of a wet-chemical etching environment.
All of these techniques are limited in various ways, but unlike the newly proposed etching method, none of them provide a crystallographically defined precision. Also because they all rely on surface masking they are limited in terms of making deep structures with high spatial resolution.
This invention provides a method of etching structural depressions in a substrate, the method comprising:
An etching process according to the invention involves a controlled inversion or alignment of ferroelectric domains in the substrate (e.g. through an electric poling process), followed by an etching of structural depressions which follows the domain-alignment boundaries.
The invention recognises that the domain structure imposed by a poling process can extend smoothly and potentially uniformly throughout the thickness of the substrate, or at least well into the substrate. The invention uses this to provide an etching method for potentially producing deep macroscopic features with precise (atomic scale) dimensions and positioning. Indeed, some of these advantages stem from the way in which the technique involves a modification of the bulk crystal at the unit cell level.
With the invention, structural depressions (e.g. greater than 1 xcexcm deep) are created. This is quite different to any previous proposals to use differential etching techniques to allow chemical or electrical features of a substrate to be visualised (just creating a surface patterning).
Further respective aspects and features of the invention are defined in the appended claims.